Laser radar systems have numerous uses in military and civilian remote sensing applications. Both direct (incoherent) and heterodyne (coherent) detection laser-radar measurements are possible. Incoherent detection acts as a "photon bucket," collecting the received photons, and is insensitive to their phase. Coherent detection optically mixes the received light field with a local oscillator field on the surface of a detector and is sensitive to the relative phases, or frequencies, of the two fields, as well as the intensity of the received light field. By employing shot-noise-limited heterodyne detection, coherent laser radar systems also have the advantage of greater sensitivity, especially when using infrared laser wavelengths. Wavelengths above 1.4 microns are more eyesafe than shorter wavelengths, and laser radar systems using such sources are hence of particular interest. By comparing the frequency of radiation which is scattered back toward the laser radar system to that of the transmitted pulse, the frequency change due to the relative motion of the scatter, with respect to the laser radar system (Doppler shift), can be computed and used to measure the component of the velocity along the line-of-sight (radial velocity). Measuring the radial velocity along 3 or more lines-of-sight allows computation of the velocity vector. Small aerosol particles which are entrained in the atmosphere move with the wind and serve as distributed scatters, allowing the remote measurement of aerosol concentrations and wind velocity using a coherent laser radar system to measure the intensity and frequency of the scattered laser light.
Coherent laser radar systems have been developed and demonstrated. U.S. Pat. No. 3,532,427 to Paine et al., describes a laser radar system for measuring fluid flow velocity. U.S. Pat. No. 3,856,402 to Low et al. describes a clear air turbulence detector utilizing a carbon dioxide coherent laser radar system. U.S. Pat. No. 3,984,685 to Fletcher et al. describes a laser radar wind measurement system using a carbon dioxide laser radar system. The majority of coherent laser radar remote sensing performed to date has utilized carbon dioxide gas laser technology at eyesafe wavelengths of 9-11 microns, however these carbon dioxide systems have several disadvantages that limit their practical use, such as cryogenically cooled radiation detectors, large size, and because of the long wavelength used, poor spatial resolution and low backscatter from atmospheric aerosol particles.
Laser radars utilizing shorter wavelengths, such as solid-state coherent laser radars operating at 1.06 microns using a Nd:YAG system, have also been demonstrated [see T. J. Kane et al., Opt. Lett. 12:239-241 (1987) and M. J. Kavaya et al., Opt. Lett. 14:776-778 (1989)]. This type of system uses a master-oscillator power-amplifier (MOPA) configuration in which a low power, narrow bandwidth laser beam is amplified by many orders of magnitude in a very high-gain multiple-pass amplifier. U.S. Pat. No. 4,902,127 to Byer and Kane describes such a 1.06 .mu.m Nd:YAG MOPA coherent laser radar system, and broadly describes the use of numerous solid-state lasers in coherent laser radar systems. Nd:YAG operating at 1.06 .mu.m, while not eyesafe, provides a very high emission cross-section which permits construction of highly efficient, high-gain laser amplifiers. A serious drawback with previously-known shorter wavelength laser radar systems, such as Nd:YAG-based systems, is that the emission wavelength of 1.06 microns is, according to the present American National Safety Institute (ANSI) standards, significantly more dangerous to the eye than wavelengths longer than 1.4 microns. Several laser ions emit at wavelengths longer than 1.4 microns, but they typically have much smaller emission cross-sections, which makes implementation of a MOPA coherent laser radar system difficult, if not impossible, due to the required high optical gain.
An alternative technique for a coherent laser radar is to use an injection-seeded oscillator to produce frequency-stable high-power optical pulses. However, such injection-seeded oscillators do not inherently possess the temporal or frequency stability needed for highly accurate range and velocity measurements. U.S. Pat. No. 4,902,127 suggests the use of injection-seeded oscillators in eyesafe laser radar systems, but does not contain a teaching of a device configuration or any methods or components which would enable sufficient temporal or frequency stability to allow the use of injection-seeded oscillators in coherent laser radar applications requiring highly accurate range and velocity measurements.